1. Field of the Invention
The present invention relates generally to medical devices and methods. More particularly, the present invention relates to radially expansible luminal prostheses, such as vascular stents and grafts.
Luminal prostheses are provided for a variety of medical purposes. For example, luminal stents can be placed in various body lumens, such as blood vessels, the ureter, urethra, biliary tract, and gastrointestinal tract, for maintaining patency. Luminal stents are particularly useful for placement in atherosclerotic sites in blood vessels or fistula or bypass grafts. Luminal grafts can be placed in blood vessels to provide support in diseased regions, such as aortic abdominal, and other aneurysms.
Both stent and graft prostheses must meet certain mechanical criteria to function successfully. In particular, such prostheses should be at least partly flexible or articulated (i.e., adjacent expansible ring segments are connected by links that articulate relative to one another) over their lengths so that they may be advanced through tortuous body lumens, such as those of the coronary vasculature. In addition, the prostheses should have controllable length change properties, either to maintain their original length or to have the ability to elongate or foreshorten, as desired, when the prostheses assume an expanded configuration. Further such prostheses must have sufficient mechanical strength, particularly hoop strength after they are expanded, in order to mechanically augment the luminal wall strength and thus maintain lumen patency. The ability to meet these requirements is severely limited in the case of stents and grafts which are delivered in a radially constrained or collapsed configuration. Such prostheses must radially expand at a target site within the body lumen, so any adaptations which are intended to enhance flexibility must not interfere with the ability to radially expand or to maintain strength once expanded.
Prior luminal prostheses often have structures which present a risk of injury as they are endoluminally delivered (i.e., tracked) to and/or released at a target site within a patient""s body lumen. In particular, many vascular stents comprise a plurality of circumferentially connected and spaced-apart ring segments which deform circumferentially as the stent is radially expanded. The Palmaz stent described in U.S. Pat. Nos. 5,102,417 and 4,776,337, is typical of such stents. Such stent designs can present challenges in both delivery and deployment. For example a phenomenon called xe2x80x9cflaringxe2x80x9d occurs when the longitudinal elements of the distal or proximal end of the prosthesis are bent outward to assume a crown-like configuration due to bending forces placed on these elements as the prosthesis passes through tortuous body passageways. Flaring can create the same deleterious effects as the previously described fish scaling phenomenon, injuring or traumatizing the blood vessel wall as the prosthesis is delivered or tracked within the blood vessel. In addition, flaring may increase a tendency for stent movement relative to a delivery balloon, thus causing an improperly deployed stent or, possibly, dislodging the undeployed stent completely from the catheter.
In addition to challenges during delivery, prior luminal prostheses can suffer problems during expansion, particularly during balloon expansion of malleable stents. For example, it has been found that balloon expansion of vascular stents often results in the ends of the stent expanding preferentially compared to the center of the stent. Such xe2x80x9cdog-bonexe2x80x9d expansion inhibits sufficient expansion of the center or ends of the stent, thus leaving a restricted luminal area in the fully deployed stent. Conversely, sometimes it will be desired to flare the ends of the stent in order to lock the stent in place and prevent the ends of the stent from collapsing after deployment. The ability to program stent expansion over the length of the stent has generally been lacking in prior stent designs.
A still further problem experienced by many prior stent designs is a lack of vessel coverage after expansion. It will be appreciated that the ability to support luminal patency and inhibit hyperplasia and other luminal in-growth can be enhanced if relative coverage of the luminal wall area by the expanded stent is increased. Thus, stent designs which afford a greater luminal wall coverage, or which minimize the free space between stent structures, while minimizing the amount of stent material used may be advantageous. Such increase of luminal wall coverage, however, should not be achieved at the expense of xe2x80x9ccrimpability.xe2x80x9d Particularly for vascular applications, it is desirable that the diameter of the stent be reduced as much as possible during delivery, e.g., when crimped over a delivery balloon. By minimizing the crimped-stent diameter, both trackability and the ability to cross smaller lesions and access more distal lesions will be enhanced. In addition, a larger crimped-stent diameter may increase the risk of stent movement relative to the deployment balloon which, in turn, could cause an improperly deployed stent or even loss of the undeployed stent from the catheter. The ability to reduce the stent diameter is generally limited by the amount of material in the stent itself. Thus, designs which increase the ability of the stent to cover the luminal wall without significantly reducing the xe2x80x9ccrimpabilityxe2x80x9d would be particularly desirable.
For these reasons, it would be desirable to provide improved stent, graft, and other luminal prostheses. In particular, it would be desirable to provide improved luminal prostheses which exhibit a high degree of flexibility with minimum losses of hoop strength and luminal wall coverage after the prostheses are expanded. For example, the design should be such that the expanded prostheses will conform to both curved and straight vessels with minimal or no straightening or other unintended deformation of the vessel wall. Such luminal prostheses should be trackable, preferably being both flexible and presenting minimum risk of injury to the luminal wall as they are being delivered. In particular, the prostheses should avoid xe2x80x9cfish scalingxe2x80x9d and should be highly xe2x80x9ccrimpablexe2x80x9d so that the prostheses diameter during delivery can be reduced. The luminal prostheses will preferably further display superior expansion characteristics. In particular, the prostheses designs should permit selective programming of the expansion characteristics along the length of the prostheses. For example, the designs should permit preferential expansion over the central portion of the prosthesis, or alternatively at either or both ends of the prostheses depending on the particular application in which the prosthesis is to be used. Still further preferably, upon expansion the prostheses should display superior luminal wall coverage and adequate to superior hoop strength in order to best maintain patency of the body lumen being treated. At least some of these objectives will be met by the luminal prostheses described and claimed hereinafter.
2. Description of the Background Art
Stents having expansible ring segments joined by sigmoidal links and axial beams are described in WO 99/17680. Stents comprising expansible rings including struts and hinges where the hinges are configured to have different opening forces are described in U.S. Pat. No. 5,922,020. EP 662 307 describes an expansible stent having serpentine elements with varying degrees of curvature to provide controlled expansion characteristics. WO 00/003662 describes a stent delivery balloon which preferentially opens a center region of a stent as the balloon is expanded. U.S. Pat. No. 6,017,365, describes a stent with serpentine segments with non-linear struts and sigmoidal links. Other patents of interest include U.S. Pat. Nos. 4,776,337; 5,102,417; 6,017,362; 6,015,429; and 6,013,854.
The present invention provides improved luminal prostheses suitable for endoluminal placement within body lumens, particularly blood vessels, and most particularly coronary and peripheral arteries. The luminal prostheses may be in the form of stents, intended for maintaining luminal patency, or may be in the form of grafts, intended for protecting or enhancing the strength of a luminal wall. Generally, the term xe2x80x9cstentxe2x80x9d will be used to denote a vascular or other scaffold structure comprising expansible components, such as ring segments, which when expanded form an open lattice or framework which is disposed against the luminal wall. In contrast, the term xe2x80x9cgraftxe2x80x9d will generally denote such as luminal scaffold which is covered by a liner, membrane, or other permeable or impermeable layer which covers at least a portion of the scaffold. The drawings included herein are generally directed at stent structures, but it will be appreciated that corresponding graft structures could be provided by incorporating a liner, membrane, or the like, on either the outer or inner surfaces of the stent.
The luminal prostheses of the present invention will be radially expansible, usually by the application of a radially outward internal force to expand a minimally resilient (usually malleable) prosthesis structure. Such radially outward internal force will usually be provided by an inflatable balloon, and such balloon expansible stents are well-known in the art and described in the background references which have been cited above and are incorporated herein by reference. Alternatively, at least some of the radially expansible luminal prostheses of the present invention may be self-expanding. By fabricating the prostheses from a resilient material, usually a metal, such as spring stainless steel, a nickel-titanium alloy (such as Nitinol(copyright) alloy), or the like, the prosthesis can be designed to have a large (fully expanded) diameter in an unconstrained state. The diameter of the prosthesis can be reduced by applying a radial constraint, e.g., by placing the prosthesis within a sleeve, tube, or other constraining structure. In that way, the self-expanding prosthesis can be delivered while constrained and deployed by releasing the constraint at the target site within the body lumen. The general principles of constructing self-expanding stents and other luminal prostheses are also well-known in the art and described in at least some of the background references which have previously been incorporated herein.
In a first aspect of the present invention, a radially expansible luminal prostheses comprises a plurality of serpentine ring segments including struts connected by hinge regions. The struts may be straight or may have non-linear configurations, e.g., being curved, wavy, or the like. The use of non-linear struts may be advantageous in order to increase the area of the strut which engages the luminal wall after expansion without significantly reducing flexibility and/or crimpability of the strut. The hinge regions are usually formed by a short curved or C-shaped region which permits the connected struts to reverse direction in order to define the serpentine ring pattern. Adjacent serpentine rings are joined by sigmoidal links, i.e., S-shaped elements which may be malleable or elastically deformable in order to allow the adjacent segments to flex relative to each other during prosthesis delivery and expansion. The sigmoidal links are attached to a side of the hinge region, typically located at the point where the hinge attaches to or transforms into the strut. The use of such sigmoidal links is beneficial since it permits the longitudinal expansion or contraction of the prosthesis to accommodate length changes as the prosthesis is expanded. Such links further permit bending of the prosthesis since they allow differential motion of adjacent serpentine rings. Such flexibility is particularly advantageous since it allows improved tracking of the prosthesis as it is delivered to an endoluminal location. The sigmoidal links also improve the conformability of the expanded prosthesis when placed in a native vessel, artificial graft, or other body lumen location. Such a structure distinguishes prior art designs where a sigmoidal link is attached at or near the apex of the link. By attaching the sigmoidal link closer to the strut, the adjacent ring segments can be positioned closer to each other. Moreover, because the links attach away from the apex of the hinge region, stress at the apex is reduced and uniform expansion of each ring segment is enhanced.
In a second aspect of the present invention, the apexes of opposed hinge regions on adjacent serpentine rings will be circumferentially offset. That is, the hinge regions apices on at least some (often all) of the serpentine rings will be aligned with the trough regions on the adjacent serpentine ring. In this way, the hinge regions are circumferentially offset so that the circumferential length of the sigmoidal links connecting proximate hinge regions can be reduced. Such a design also allows for adjacent serpentine rings to be closer together to allow for improved vessel coverage upon prosthesis expansion. Such a design also permits an increase in the diameter of the curved portions of the sigmoidal link which further improves stress distribution and opening characteristics of the prosthesis. Preferably, the luminal prostheses of the present invention will both have the sigmoidal links attached to the sides of the hinge regions and have the hinge regions circumferentially offset in order to achieve the greatest improvement in flexibility, crimpability, and uniform expansion characteristics.
The sigmoidal links will preferably have a S-shaped geometry with two outer connecting legs joined to a central leg by U-shaped joints. In some instances, it might also be possible to provide Z-shaped sigmoidal links, but those will generally be less preferred. In connecting the sigmoidal links to the hinge regions of the serpentine rings, the outer connecting legs will generally be oriented in the annular or circumferential direction and attach to the hinge region on its side.
In a further aspect of the present invention, a radially expansible luminal prosthesis comprises a plurality of ring segments which are expansible in response to a radially outward force. The ring segments may comprise serpentine rings, as generally described above, or may comprise zig-zag segments, box segments, or other conventional prosthesis ring patterns. The expansion characteristics of the luminal prosthesis may be varied over the length of the prostheses by controlling or programming the characteristics of each of the adjacent expansible ring segments. For example, different ring segments can be controlled to have different cross-sectional areas, e.g., differing widths, thicknesses or both, so that the amount of radially outward force needed to open the stent is lesser or greater. Alternatively, in the case of serpentine or zig-zag ring patterns, the strut length may be varied in order to control the force needed to open the stent. That is, ring segments having a greater strut length will open with a lesser force since the increased strut length will leverage the force applied to a hinge region so that the hinge region will open sooner. Other techniques for controlling the expansion characteristics of an individual ring segment may also be employed, such as those described in U.S. Pat. No. 5,922,020, the full disclosure of which has previously been incorporated herein by reference. Depending on the objective, the ring segments near either or both ends of the prosthesis may be programmed to open more or less readily so that, when applying a constant radially outward force along the length of the prosthesis, the stent will first open either at both ends or in the middle. It will be appreciated that by employing balloons which also have variable expansion characteristics, such as those described in WO 00/03662, a wide variety of prosthesis expansion characteristics can be provided over the length of the prosthesis.